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Abstract:

A piston 1 is employed in an engine 50 where a tumble flow is generated
within a combustion chamber 53. The piston 1 includes: a top ring channel
3; and a portion 10 of an outer circumferential portion of a top surface,
the portion 10 positioned opposite to an adjacent cylinder of the engine
50, the portion 10 having a raised shape so as not to expose a position
P, of a bore wall surface, facing the top ring channel 3 face in the top
dead center during at least a period from a time when the piston 1 is
positioned at a compression top dead center to a time when a quantity of
heat transfer in the combustion chamber 53 is the highest.

Claims:

1. A piston of an engine, the engine employed as a multicylinder engine
in which rotational flow is generated in a combustion chamber, the piston
comprising: a top ring channel; and a portion of an outer circumferential
portion of a top surface of the piston, the portion positioned opposite
to an adjacent cylinder of the multicylinder engine, the portion having a
raised shape so as not to expose a position, of a bore wall surface,
facing the top ring channel face in the top dead center during at least a
period from a time when the piston is positioned at a compression top
dead center to a time when a quantity of heat transfer in the combustion
chamber is the highest.

2. The piston of the engine of claim 1, wherein: the rotational flow is
tumble flow; and the period from the time when the piston is positioned
at the compression top dead center to the time when the quantity of heat
transfer in the combustion chamber is the highest includes a period from
the time when the piston is positioned at the compression top dead center
to the time when a crank angle is a given degree from 30 degrees to 50
degrees as setting the compression top dead to an origin, in the forming
of the portion.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a piston of an engine, and more
particularly, to the piston of a multicylinder engine in which rotational
flow is generated in a combustion chamber.

BACKGROUND ART

[0002] There is conventionally known an engine in which rotational flow
such as tumble flow or swirl flow is generated in a combustion chamber.
In such an engine, the strong rotational flow is generated, thereby
increasing the turbulence of the mixed gas. This can improve the
combustion speed, and the high speed combustion can improve the mileage.
In this regard, for example, Patent Document 1 discloses a technique
relevant to an engine in which the tumble flow is generated and relevant
to the present invention. In another piston, for example Patent Document
2 or 3 discloses a technique of a structure relevant to the present
invention is disclosed.

[0006] Incidentally, the temperature of, more particularly, a wall portion
formed between cylinders tends to be increased, due to the structure of
the multicylinder engine. Specifically, as illustrated in FIG. 8, the
temperature of the wall portion illustrated in (a) is higher than the
temperature of the wall portion illustrated in (b), in the range of all
of the engine driving states. Also, when the engine driving state is
changed from a low speed and low load state to a high speed and high load
state, the degree where the temperature illustrated in (a) is increased
is larger than the degree where the temperature illustrated in (b) is
increased. In this regard, the increase in the temperature of the wall
portion formed between the cylinders is considered to abnormally consume
an engine oil. In particular, there is a concern that this occurs in the
high speed and high load driving state in the engine of the high speed
combustion. Further, the increase in the temperature might obstruct the
improvement in the mileage, in particular, in the engine of the high
speed combustion, in order to improve the mileage.

[0007] The present invention has been made in view of the above
circumstances and has an object to provide a piston of an engine, thereby
suitably suppressing the increase in the temperature of a wall portion
formed between cylinders of the multicylinder engine.

Means for Solving the Problems

[0008] The present invention to solve the above problem is a piston of an
engine, the engine employed as a multicylinder engine in which rotational
flow is generated in a combustion chamber, the piston including: a top
ring channel; and a portion of an outer circumferential portion of a top
surface of the piston, the portion positioned opposite to an adjacent
cylinder of the multicylinder engine, the portion having a raised shape
so as not to expose a position, of a bore wall surface, facing the top
ring channel face in the top dead center during at least a period from a
time when the piston is positioned at a compression top dead center to a
time when a quantity of heat transfer in the combustion chamber is the
highest.

[0009] In the present invention, it is preferable that the rotational flow
should be tumble flow and the period from the time when the piston is
positioned at the compression top dead center to the time when the
quantity of heat transfer in the combustion chamber is the highest should
include a period from the time when the piston is positioned at the
compression top dead center to the time when a crank angle is a given
degree from 30 degrees to 50 degrees as setting the compression top dead
to an origin, in the forming of the portion.

Effects of the Invention

[0010] According to the prevent invention, a temperature of a wall portion
formed between cylinders of a multicylinder engine is suitably
suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a schematic view of an engine;

[0012]FIG. 2 is a horizontal sectional view of a substantial parts of the
engine;

[0013] FIG. 3 is a perspective view of specifically illustrating a piston
of the engine;

[0014] FIG. 4 is a sectional view of the piston of the engine taken along
line A-A illustrated in FIG. 3;

[0015] FIG. 5 is an explanatory view of the piston of the engine;

[0016]FIG. 6 is a view of a quantity of heat transferred in a combustion
chamber;

[0017]FIG. 7 is a view of the quantities of heat transferred in the
combustion chamber depending on the tumble ratio; and

[0018] FIG. 8 is a view of an example of a temperature in the vicinity of
a cylinder depending on an engine driving state.

MODES FOR CARRYING OUT THE INVENTION

[0019] In the following, embodiments will be described in detail with
reference to the drawings.

[0020] An engine 50 illustrated in FIGS. 1 and 2 is a multicylinder engine
with four inline cylinders, and is equipped with a piston 1 of the engine
according to the present embodiment (hereinafter, simply referred to as
piston), in addition to a cylinder block 51, and a cylinder head 52, an
intake valve 55, an exhaust valve 56, and a spark plug 57. The cylinder
block 51 is formed with plural (here, four) cylinders 51a and a water
jacket 51b. A wall portion 51c is formed between the adjacent cylinders
among the plural cylinders 51a. The piston 1 is housed in the cylinder
51a. The cylinder head 52 is secured to a top surface of the cylinder
block 51. A combustion chamber 53 is defined as a space surrounded by the
piston 1, the cylinder block 51, and the cylinder head 52.

[0021] The cylinder head 52 is provided with an intake port 52a and an
exhaust port 52b. The intake port 52a introduces an intake air S to the
combustion chamber 53, and the exhaust port 52b exhausts a gas in the
combustion chamber 53. The intake port 52a corresponds to an intake air
introduction portion for introducing an intake air to generate rotational
flow in the combustion chamber 53. The intake air S introduced in the
combustion chamber 53 forms tumble flow T. In this regard, the tumble
flow T is generated with a high tumble ratio (which is the rotation
number of the tumble flow T, while the piston 1 is reciprocated once)
about 2.0 in the engine 50. The tumble ratio is estimated by AVL
simulation. The cylinder head 52 is provided with the intake valve 55 and
an exhaust valve 56 for opening and closing respectively the intake port
52a and the exhaust port 52b. Further, the cylinder head 52 is provided
with the spark plug 57 projecting on an upper substantial central portion
of the combustion chamber 53.

[0022] Next, the piston 1 will be described. The piston 1 is provided at
its upper surface with a cavity 2 guiding the tumble flow T as
illustrated in FIGS. 3 and 4. The cavity 2 is provided for guiding the
tumble flow T in the direction of a line passing through the exhaust side
and the intake side within the combustion chamber 53. The piston 1 is
provided at its circumferential portion with plural (here, three) ring
channels. Among them, the ring channel closer to the top surface is a top
ring channel 3. Piston rings (not illustrated), respectively installed
into the ring channels including the top ring channel 3, each have
functions to scrape down an oil on a wall surface of the cylinder 51a as
a boa wall surface and to maintain air proof of the combustion chamber
53.

[0023] In addition, the piston 1 is formed with a pin boss hole 4.
Portions 10 are positioned respectively at both ends in the extending
direction of the pin boss hole 4 in an outer circumferential portion of
the top surface of the piston 1. Each portion 10 does not have a flat
shape but rather a raised shape. Specifically, each portion 10 is formed
in such a shape to be gradually raised from both of the intake side and
the exhaust side. At least one of the portions 10 is arranged in such a
position to face an adjacent cylinder of the engine 50. That is, at least
one of the portions 10 is arranged to face the wall portion 51c.

[0024] As illustrated in FIG. 5, within the combustion chamber 53, the
portions 10 are formed as follows. Herein, the piston 1 is illustrated by
a solid line in cases where a crank angle is 40 degrees ATDC, and the
piston 1 positioned at the top dead center is illustrated by a dashed
line in FIG. 5. Also, the position P indicates the position, of the wall
surface of the cylinder 51a, facing the top ring channel 3 in the top
dead center. Each portion 10 is formed into such a shape as not to expose
the position P of the wall surface of the cylinder 51a during at least a
period from the time when the piston 1 is positioned at a compression top
dead center to the time when the heat flux indicating the quantity of
heat transfer in the combustion chamber 53 is the highest. In this
regard, the increase in the temperature of a portion 51ca, lower than the
position P, of the wall portion 51c facing the portions, particularly,
the portion 10 have to be suppressed in light of the suppression of the
abnormal consumption of the oil caused by rising the oil.

[0025] On the other hand, a heat flux changes as illustrated in FIG. 6 in
the engine 50. As illustrated in FIG. 6, the heat flux drastically rises
just after the compression top dead center, the heat flux becomes peak,
and so gradually falls afterward. In this regard, specifically, the heat
flux is the highest when the crank angle is about 25 degrees, the heat
flux is zero when the crank angle is about 50 degrees ATDC afterward. If
the portion 51ca is made not to be exposed while the heat flux is, being
generated, the increase in the temperature of the portion 51ca, caused by
exposing the portion 51ca to flames or combustion gases, can be
suppressed.

[0026] For this reason, in order to suppress the increase in the
temperature of the portion 51ca, in the forming of the portions 10, it is
suitable that the piston 1 should not expose the position P of the wall
surface of the cylinder 51a during at least a period from the time when
the piston 1 is positioned at the compression top dead center to the time
when the heat flux is the highest (herein, 25 degrees ATDC). Further, in
forming the portions 10, in light of the change manner in the heat flux
as illustrated in FIG. 6, it is preferable that the period from the time
when the piston 1 is positioned at the compression top dead center to the
time when the heat flux is the highest should include a period from the
time of the compression top dead center to a time of a given degrees of a
crank angle (from 30 degrees to ATDC 50 degrees ATDC) as setting the
compression top dead center to an origin.

[0027] In this regard, the given angle is set to be 30 degrees, whereby a
range R of the crank angle suppressing the heat transfer to the portion
51ca includes the region where the heat flux is particularly higher
around the peal value of the heat flux illustrated in FIG. 6 (from 20
degrees ATDC to 30 degrees ATDC). Also, the given angle is set to 50
degree, whereby the range R can include the heat flux illustrated in FIG.
6.

[0028] On the other hand, when the shapes of the portions 10 are formed to
be larger, the strengths of the portions 10 might be influenced, and
besides the piston 1 might be increased in weight. In this regard, the
heat flux mainly increases until 40 degrees ATDC as illustrated in FIG.
6. For this reason, in the forming of the portions 10, in light of the
change manner in heat flux illustrated in FIG. 6, it is preferable that
the given angle should be set to 40 degrees. In this regard, the given
angle is set to 40 degrees, thereby further suppressing the heat transfer
to the portion 51ca as compared with cases where the given angle is set
to 30 degrees. Additionally, the portions 10 can be reduced in size as
compared with cases where the given angle is set to 50 degrees.

[0029] On the other hand, the heat flux changes depending on the tumble
ratio as illustrated in FIG. 7. As illustrated in FIG. 7, the crank angle
of the heat flux peak is gradually spaced away from the compression top
dead center as the tumble ratio (TR) is lower. Also, the heat flux peak
value is gradually lower as the tumble ratio is lower. In this regard, in
cases where a given angle is set to 40 degrees, the heat flux peak can be
included in the range R, not only a case (T1) where the tumble ratio is
high (specifically, 2.0) but also a case (T2) where that is middle
(specifically, 1.2) and a case (T3) where that is lower (specifically,
0.5). Further, in cases where a given angle is set to 40 degrees, in
particular, even in the case T2, the heat transfer to the portion 51ca
can be suitably suppressed together with the decrease in the peak value
of the heat flux.

[0030] For this reason, in cases where a given angle is set to 40 degrees,
the adjustability to a wide range including the high tumble ratio can be
enhanced.

[0031] On the other hand, the crank angle where the heat flux peak is
generated comes gradually closer to the compression top dead center as
the tumble ratio is higher. Also, the heat flux peak value becomes
gradually higher as the tumble ratio is higher. In this regard, when the
tumble ratio is set to be higher than 2.0,a given angle is set to be
smaller than 40 degrees depending on the tumble ratio. Therefore, the
portions 10 can be further reduced in size, as compared with cases where
a given angle is set to 40 degrees, while the heat transfer is being
suppressed to the same extent in cases where a given angle is set to 40
degrees.

[0032] Also, in cases of the tumble ratio lower than 2.0, the heat flux
peak value is lower than that of the tumble ratio 2.0. However, a given
angle is set to be greater than 40 degrees, thereby further suppressing
the heat transfer as compared with cases where a given angle is set to 40
degrees.

[0033] Also, in the engine 50, the tumble flow T is generated as the
rotational flow in the combustion chamber 53 to be maintained to the
latter half of the compression stroke, and is them collapsed. This
disturbs the atmosphere in the combustion chamber 53, thereby improving
the combustion speed to perform the high speed combustion. In this
regard, in the engine 50 performing the high speed combustion, the
improvement in the combustion speed increases the temperature of the
combustion gas, and the rotational flow causes the thermal boundary layer
to be thin. As a result, the heat transfer coefficient becomes large,
whereby the temperature of the wall surface of the combustion chamber 53
becomes higher. Also, in the engine 50 performing the high speed
combustion, the more rotational flow is strengthened, the more calorific
value per unit time increases as the rotational number and the load are
higher. Therefore, the heat transfer coefficient becomes much higher.
That is, in the engine 50 generating the rotational flow in the
combustion chamber 53 and performing the high speed combustion, the above
circumstances raise a problem with, in particular, the increase in the
temperature of the wall portion 51c. In this regard, the piston 1 which
can suppress the increase in the temperature of the portions 51ca is
suitable for the engine 50 generating the rotational flow in the
combustion chamber 53 and performing the high speed combustion.

[0034] While the exemplary embodiments of the present invention have been
illustrated in detail, the present invention is not limited to the
above-mentioned embodiments, and other embodiments, variations and
modifications may be made without departing from the scope of the present
invention. For example, the intake port 52a has been described as an
intake air introduction portion in the above embodiment. However, the
present invention is not limited to these arrangements. For example, the
intake air introduction portion may be achieved by a flow control valve,
which is provided within the intake port to control the flow of the
intake air, or by the combination of the flow control valve and the
intake air port. Also, the tumble flow T has been described as the
rotational flow in the above embodiment. However, the present invention
is not limited to this. The rotational flow may be swirl flow or skew
tumble flow.